Co2 sorption by supported amino acid ionic liquids

ABSTRACT

The present invention concerns the absorption and desorption behaviour of carbon dioxide (CO 2 ) using ionic liquids derived from amino acids adsorbed on porous carrier materials.

FIELD OF THE INVENTION

The present invention concerns the absorption and desorption behaviour of carbon dioxide (CO₂) using ionic liquids derived from certain amino acids, adsorbed on porous carrier materials.

BACKGROUND OF THE INVENTION

Carbon dioxide is recognized as a greenhouse gas that contributes to climate changes. A major cause of CO₂ emission is from combustion of fossil fuels like oil and coal in production of electricity and heat. The world consumption of coal is expected to increase by 49% in 2030 and accordingly much attention has been focused on reducing CO₂ emission from power plant flue-gas streams. Currently, aqueous solutions of organic amines are being used to capture CO₂ in scrubbers as carbamates despite concerns about, e.g. low absorbent capacity and energy intensive regeneration by desorption, which may require up to one-third of the total energy output from the power plant. Other technical challenges with amine sorbents include corrosion of steel pipes and pumps as well as thermal and chemical decomposition. In addition concern has been expressed about emission of the amines to the atmosphere leading to serious human health hazards. Solid absorbers also work unsatisfactorily among others due to the high desorption energy required to desorb CO₂ and regenerate the absorber.

In order to overcome these inherent problems, it is highly desired to develop a viable and energy-efficient technology for CO₂ capture by use of alternative, suitable absorbents.

Ionic liquids (ILs) are promising candidates as absorbents in CO₂ removal due to their relatively high thermal stability, exceptionally low vapour pressure and tuneable physicochemical properties [see eg. a) F. Jutz et al. Chem. Rev. 2011, 111, 322; b) J. Huang et al. Aust. J. Chem. 2009, 62, 298]. There have been many reports on CO₂ capture using common ILs (typically referred to as first-generation ionic liquids), and a maximum CO₂ absorption capacity of 0.75 in mole fraction of CO₂ has been found for [C₈MIM][PF₆] at 40° C. under 93 bar pressure [see eg. a) L. A. Blanchard, Z. Gu, Joan F. Brennecke, J. Phys. Chem. B 2001, 105, 2437; b) Z. Gu, Joan F. Brennecke, J. Chem. Eng. Data 2002, 47, 339; c) Sudhir N. V. K. Aki, Berlyn R. Mellein, Eric M. Saurer, Joan F. Brennecke, J. Phys. Chem. B 2004, 108, 20355]. The CO₂ absorption capacity of these kinds of ILs is, however, limited due to the relatively weak physisorption taking place between the IL and CO₂. To circumvent this drawback, Davis and co-workers developed task-specific ionic liquids (TSILs) which are able to chemically bind CO₂ to amine-functionalised imidazolium-based IL at ambient conditions. Even though these TSILs form a chemical bond with CO₂ the absorption capacity was only 0.5 mol of CO₂ per mol of IL (mole ratio of 0.33) due to intermolecular carbamate formation (1:2 mechanism), [see Eleanor D. Bates et al., J. Am. Chem. Soc. 2002, 124, 926]. Zhang et al. have proposed a new mechanism for the formation of carbamic acid after CO₂ absorption in phosphonium-based amino acid functionalised ILs [J. Zhang, S. Zhang, K. Dong, Y. Zhang, Y. Shen, X. Lv. Chem. Eur. J. 2006, 12, 4021]. In this mechanism, one mol of CO₂ also reacted with two moles of IL, however in presence of water (1 wt. %) the IL could absorb an equimolar amount of CO₂ via a bicarbonate mechanism (mole ratio of 0.5). Recently, the formation of carbamic acid in amino acid-based ILs was further confirmed by Brennecke and co-workers by examining proline and methionine functionalised phosphonium-based ILs for CO₂ absorption. Here it was shown that one mol of IL can absorb one mol of CO₂ (1:1 mechanism) [see B. E. Gurkan, J. C. de la Fuente, E. M. Mindrup, L. E. Ficke, B. F. Goodrich, E. A. Price, W. F. Schneider, J. F. Brennecke, J. Am. Chem. Soc. 2010, 132, 2116]. A few other reports have also published equimolar amount of CO₂ absorption in functionalised ILs, but the typical CO₂-absorption capacity for published IL based solutions is sub-stoichiometric rather than super-stoichiometric.

Chemical absorption (chemisorption) of CO₂ was first observed in imidazolium acetates, where the CO₂ can be trapped either as bicarbonate or, as recently found, also as a carboxylate on carbon position 2 of the imidazole via a carbene mechanism [U.S. patent application Ser. No. 10/737,090; U.S. Pat. No. 5,336,298; Maginn, E., DOE Report, quarterly, 01/05-03/05 2005, 1-12, DOE Scientific and Technical Information, Oak Ridge, Tenn.]. This was recently proven by Rogers and co-workers upon determination of its crystal structure [Gurau G et al. Angew Chem Int Ed, 2011, 50:12024-12026]. However, amine functionalized ILs have proven to have a higher CO₂ uptake stoichiometry and has attracted more attention. If an amine functionality is linked to the cation, intermolecular carbamate formation takes place resulting in a maximum stoichiometry of 0.5 moles of CO₂ per mol of IL [^(a)Galan Sanchez L et al., Solvent properties of functionalized ionic liquids for CO₂ absorption. Chem Engineer Res Design, 2007, 85: 31-39; ^(b)Soutullo M et al. Reversible CO₂ capture by unexpected plastic-, resin-, and gel-like ionic soft materials was discovered during the combi-click generation of a TSIL library. Chem Mater, 2007, 19: 3581-3583]. This has been demonstrated for imidazolium-based cations, where the amine is introduced on one of the alkyl chains. A major drawback of this method is that the already high viscosity of the IL becomes much higher when attaching another functional group to the IL. The important point is, however, that these compounds show much higher CO₂ uptake than their non-functionalized counterparts.

If the amine functionality is attached to the anion of the IL, carbamate species also form, but there is a possibility that the negatively charged functionality on the anion can take up the proton released upon CO₂ capture forming carbamic acid. Then CO₂ can be absorbed with a stoichiometry of up to 1:1, meaning much more efficient use of the IL. However, in most cases, the absorption capacity still corresponds to 0.5:1. This could very well be related to residual water in the IL interfering with the released proton, preventing it from protonating the anionic part.

Therefore it must be concluded that despite growing demand, there has not yet been suggested a satisfactory and economically feasible technical solution to the problem of removing CO₂ from eg. flue gases which has a high sorption efficiency, a high capacity and a sufficient robustness to allow for uninterrupted performance over long time periods.

SUMMARY OF THE INVENTION

It has surprisingly been found that compositions comprising ionic liquids (ILs) based on simple tetraalkylammonium salts of certain amino acids adsorbed on porous (including mesoporous) materials are excellent absorbers of CO₂ having high sorption efficiencies. Thus, absorption of stoichiometric amounts of CO₂ at ambient temperature and pressure (ie. 1 mol CO₂ per mol IL), and in some cases even super-stoichiometric absorption was observed (i.e. more than 1 mol CO₂ per mol IL).

In a first aspect the present invention therefore provides a composition for absorption/desorption of gaseous carbon dioxide (CO₂), which comprises an ionic compound [A⁺][B⁻] and a porous material, wherein

-   -   [A⁺] is an ammonium ion of Formula I

-   -   wherein R₁, R₂, R₃ and R₄ are individually selected from linear         or branched C_(n) alkyl chains where n is an integer having a         value of at least 4 and not more than 30, and wherein     -   [B⁻] is an anion of Formula II:

-   -   wherein R₅ is selected from hydrogen or a C₁-C₆ linear or         branched alkyl group, and G can be hydrogen or a group of         Formula III:

-   -   -   wherein             -   R₆ is selected from hydrogen or a C₁-C₆ linear or                 branched alkyl group;             -   R₇ can be a C₁-C₆ linear or branched alkyl group or an                 aromatic group, and             -   when R₅ is hydrogen, R₇ may further also be attached to                 the nitrogen atom to which R₅ is attached, thereby                 forming a 4-7 membered ring together with said nitrogen.

In a second aspect the invention provides a method for absorption of gaseous CO₂ from a gas stream, which method comprises contacting said gas stream with a composition according to the first aspect of the invention.

Reversible CO₂ absorption/desorption was further demonstrated at 80° C. without degradation of the supported ionic compound (also referred to in the following as the supported ionic liquid phase (SILP) material), and consecutive absorption-desorption cycles (desorption done at 80° C. for 2 h under argon flow) demonstrated complete desorption between each sorption cycle along with preservation of absorption capacities of above 1 mol CO₂/mol IL for the second and third cycle, respectively.

In a third aspect the invention therefore also allows for desorption of the absorbed CO₂ by increasing the temperature of the composition and/or decreasing the total pressure surrounding it and/or by flushing it with a gas stream with no or lower CO₂ content than the gas stream originally applied for the absorption.

FIGURES AND TABLES

Table 1: CO₂ absorption at room temperature and structure of ionic liquids.

Table 2: CO₂ absorption capacities for selected SILP absorbers

Table 3: SILP loading and CO₂ absorption by a series of [N₆₆₆₆][Pro] materials

Table 4: Recycling CO₂ absorption capacities for [N₆₆₆₆][Pro] SILP loading 40 wt %.

Table 5: Recycling CO₂ absorption capacities for [N₆₆₆₆][Gly] SILP loading 40 wt %

FIG. 4 Shows the difference in CO₂ outlet concentration in flow of empty reactor (left curve) and with 40 wt % [N6666][Pro] SILP absorber (right curve) at 25° C. and 0.09 bar CO₂/He and total gas flow of 55 mL/min.

FIG. 5 FT-IR spectrum of [N6666][Asn] as the neat ionic liquid.

FIG. 6 FT-IR spectrum of [N6666][Asn] as the SILP version.

FIG. 7 FT-IR spectrum of [N6666][Asn] as the SILP version after CO₂ absorption.

FIG. 8 shows Absorp 3.2 TGA program

FIG. 9 shows Absorp 4.0 TGA program

FIGS. 10 a and 10 b shows recycling curves for [N6666][Gly]. FIG. 10 b shows all the cycles in one graph.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found that compositions comprising ionic liquids (ILs) based on simple tetraalkylammonium salts of certain amino acids adsorbed on high area, porous (including mesoporous) inert materials are excellent absorbers of CO₂ having high sorption efficiencies. Thus, adsorption of stoichiometric amounts of CO₂ at ambient temperature and pressure (ie. 1 mol CO₂ per mol IL), and in some cases even super-stoichiometric absorption was observed (ie. more than 1 mol CO₂ per mol IL). All superstoichiometric uptake amounts must be explained by physical absorption (physisorption) as the ILs of the present invention are only able to bind at most one mol CO₂ by a chemical reaction with the amino group.

In a first aspect the present invention therefore provides a composition for absorption/desorption of gaseous carbon dioxide (CO₂), which comprises an ionic compound [A⁺][B⁻] and a porous material, wherein

-   -   [A⁺] is an ammonium ion of Formula I

-   -   wherein R₁, R₂, R₃ and R₄ are individually selected from linear         or branched C_(n) alkyl chains where n is an integer having a         value of at least 4 and not more than 30, and wherein     -   [B⁻] is an anion of Formula II:

-   -   wherein R₅ is selected from hydrogen or a C₁-C₆ linear or         branched alkyl group, and G can be hydrogen or a group of         Formula III:

-   -   -   wherein             -   R₆ is selected from hydrogen or a C₁-C₆ linear or                 branched alkyl group;             -   R₇ can be a C₁-C₆ linear or branched alkyl group or an                 aromatic group, and             -   when R₅ is hydrogen, R₇ may further also be attached to                 the nitrogen atom to which R₅ is attached, thereby                 forming a 4-7 membered ring together with said nitrogen.

Throughout the present application the following abbreviations will be used: [N4444]: tetrabutylammonium, [N6666]: tetrahexylammonium, [N66614]: trihexyltetradecylammonium, [Met]: L-Methionate, [Ile]: L-Isoleucinate, [Gly]: Glycinate, [Asn]: L-Asparaginate, [Tyr]: L-Tyrosinate, [Pro]: L-Prolinate.

In one embodiment R₁, R₂, R₃ and R₄ are alkyl chains of identical length.

In one embodiment R₁ is selected from butyl and hexyl, and R₂, R₃ and R₄ are individually selected from butyl and hexyl.

In another embodiment R₁ is a C_(n) alkyl chain where n is an integer having a value of between 10 and 20, and R₂, R₃ and R₄ are alkyl chains of identical length selected from butyl, pentyl, hexyl, heptyl and octyl.

In a further embodiment R₁ is a C_(n) alkyl chain where n is an integer having a value of between 10 and 20, and wherein R₂, R₃ and R₄ are individually selected from butyl, pentyl, hexyl, heptyl and octyl.

In one embodiment R₇ is selected from methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, t-butyl, 1-pentyl, 2-pentyl, 3-pentyl, 1-hexyl, 2-hexyl or 3-hexyl.

In another embodiment R₇ is an aromatic group selected from benzene, substituted phenyl groups and heteroaromatic groups such as imidazole, pyridine and indole.

In a further embodiment R₇ is an aromatic group selected from the following structures wherein the asterisk (*) denotes the attachment point

In preferred embodiments [A⁺] is an ammonium ion selected from tetrabutylammonium, tetrahexylammonium, and trihexyltetradecylammonium.

In one embodiment G is hydrogen, and [B⁻] becomes an anion of this structure:

-   -   wherein R₅ is selected from hydrogen or a C₁-C₆ linear or         branched alkyl group.

In another embodiment G is a group of Formula III, and [B⁻] becomes an anion of this structure:

wherein

-   -   R₆ is selected from hydrogen or a C₁-C₆ linear or branched alkyl         group;     -   R₇ can be a C₁-C₆ linear or branched alkyl group or an aromatic         group, and     -   the dotted line denotes that when R₅ is hydrogen, R₇ may be         attached to the nitrogen atom to which R₅ is attached, thereby         forming a 4-7 membered ring together with said nitrogen.

In a preferred embodiment R₅ is hydrogen, R₆ is selected from hydrogen or a C₁-C₆ linear or branched alkyl group and R₇ is an ethyl group further attached to the nitrogen atom to form a 5-membered ring, such that [B⁻] becomes an anion of this structure:

In another preferred embodiment R₅ is selected from hydrogen or a C₁-C₆ linear or branched alkyl group, R₆ is methyl and R₇ is an ethyl group, such that [13] becomes an anion of this structure:

In further embodiments [B⁻] is an anion of one of the following amino acids:

Good to excellent absorption of CO₂ was observed for the following ionic compounds of the invention: [N4444][Met], [N6666][Ile], [N6666][Gly], [N6666][Asn], [N6666][Pro] and [N6666][Tyr], all impregnated on hydrophobic SiO₂ (Table 1 and 2).

In a preferred embodiment [B⁻] is thus an anion selected from L-Methionate, L-Isoleucinate, Glycinate, L-Asparaginate, L-Tyrosinate and L-Prolinate.

In a particularly preferred embodiment, [B⁻] is an anion selected from L-Isoleucinate, Glycinate and L-Prolinate.

In one preferred embodiment, [A⁺][B⁻] is [N6666][Ile].

In another preferred embodiment [A⁺][B⁻] is [N6666][Gly].

In a particularly preferred embodiment, [A⁺][B⁻] is [N6666][Pro].

In one preferred embodiment, [A⁺][B⁻] is [N66614][Pro].

In an embodiment the porous material is selected from porous (including mesoporous) inorganic, carbonaceous or a polymeric materials such as, but not limited to, silica, including mesoporous silica, controlled pore glass, alumina, titania (anatase), zirconia, alkaline earth metal oxides, clays and zeolites, or mixtures hereof.

In separate embodiments the porous material is selected from PG75-120 (mesh size 80-120, mean pore diameter 74 Å, pore distribution ±7:0%, pore volume 0.47 cc/g, surface area 152.7 m²/g), PG500-80 (mesh size 20-80, mean pore diameter 547 Å, pore distribution ±6:3%, pore volume 1.06 cc/g, surface area 44.5 m²/g), PG2000-200 (mesh size 120-200, mean pore diameter 1932 Å, pore distribution ±7:3%, pore volume 0.86 cc/g, surface area 11.32 m²/g) and Silica gel 90 (mesh size 80-120, mean pore diameter 74 Å, pore distribution ±7:0%, pore volume 0.47 cc/g, surface area 152.7 m²/g).

In an embodiment the composition according to the first aspect of the invention contains between 5-45% w/w of the ionic compound [A⁺][B⁻], such as 10% w/w, 20% w/w, 30% w/w or 40% w/w.

In another embodiment the composition according to the first aspect of the invention contains between 5-45% w/w, such as 10% w/w, 20% w/w, 30% w/w or 40% w/w, of an ionic compound selected from [N4444][Met], [N6666][Ile], [N6666][Gly], [N6666][Asn], [N6666][Pro] and [N6666][Tyr].

In yet another embodiment the composition according to the first aspect of the invention contains about 40% w/w of an ionic compound selected from [N6666][Ile], [N6666][Gly] and [N6666][Pro].

In specific embodiments, the composition of the first aspect has a total CO₂ absorption capacity of over about 1 mol CO₂/mol ionic liquid, such as over 0.9 mol CO₂/mol ionic liquid, over 1.1 mol CO₂/mol ionic liquid, over 1.2 mol CO₂/mol ionic liquid, over 1.3 mol CO₂/mol ionic liquid, over 1.4 mol CO₂/mol ionic liquid, over 1.5 mol CO₂/mol ionic liquid, over 1.6 mol CO₂/mol ionic liquid or over 1.7 mol CO₂/mol ionic liquid.

In other specific embodiments, the composition of the first aspect has a total CO₂ absorption capacity of over about 3% w/w CO₂, such as over 3.4% w/w CO₂, over 3.6% w/w, over 3.7% w/w, over 4.4% w/w, over 4.5% w/w or over 4.6% w/w.

In a second aspect the invention provides a method for absorption of gaseous CO₂ from a gas stream, which method comprises contacting said gas stream with a composition according to the first aspect of the invention.

In an embodiment the composition of the first aspect of the invention has an absorption capacity of over 1 mol CO₂/mol of the ionic compound [A⁺][B⁻] at a CO₂ concentration of less than 0.1 bar.

In a further embodiment the invention provides a method according to the second aspect of the invention, wherein the absorption takes place from a gas stream which contains 1-100 mol % CO₂ with an absorption capacity of over 1 mol CO₂/mol of the ionic compound [A⁺][B⁻].

In a particular embodiment, the contact between the gas stream and the absorbent composition of the invention takes place in a suitable reactor which could be a fixed-bed or movable-bed reactor using an appropriately shaped form of the composition of the invention to provide low resistance to the gas stream and thus an accompanying acceptable pressure drop across the reactor.

In particular embodiment, the gas stream is a flue-gas stream generated from combustion by large point stationary sources such as power plants.

In specific embodiments, the gas stream contains less than about 15% CO₂ such as 15% CO₂, 14% CO₂, 13% CO₂, 12% CO₂, 11% CO₂, 10% CO₂, 9% CO₂ or between 10-15% CO₂.

In a further particular embodiment, the gas stream contains significant amounts of water vapor, such as 3-5% H₂O, 5-7% H₂O, 7-10% H₂O, 10-12% H₂O, 12-14% H₂O 14-16% H₂O or more than 16% H₂O.

The compositions of the invention may be part of a composite material e.g. combined with fiber materials or other materials which improve the mechanical strength, but do not lower gas diffusion severely.

Reversible CO₂ absorption/desorption was further demonstrated at 80° C. without degradation of the supported ionic compound (also referred to in the following as the supported ionic liquid phase (SILP) material), and consecutive absorption-desorption cycles (desorption done at 80° C. for 2 h under argon flow) demonstrated complete desorption between each sorption cycle along with preservation of absorption capacities of around or above 1 mol CO₂/mol IL for the second and third cycle, respectively.

Consecutive absorption-desorption cycles for the 40 wt % [N6666][Pro] SILP absorber (desorption done at 80° C. for 2 h under argon flow) demonstrated further complete desorption between each sorption cycle, and preservation of good absorption capacities of 1.05 and 1.17 mol CO₂/mol IL for the second and third cycle, respectively (see table 4).

FIG. 10 a shows recycling curves for [N6666][Gly] and FIG. 10 b shows all the cycles shown in one graph. A very small loss of capacity is seen from the first to subsequent cycles. The absorption stoichiometry is around 1.00 mol of CO₂ per mol IL in this material. Absorption kinetics is very fast.

In a third aspect the invention also provides a method for desorption of the absorbed CO₂ by increasing the temperature of the saturated composition and/or decreasing the total pressure surrounding it and/or by flushing it with a gas stream with no or lower CO₂ content than the gas stream originally applied for the absorption.

Experimental

All chemicals were purchased from Sigma-Aldrich in p.a. quality and used as received. Glass, controlled pore series: PG75-120 (mesh size 80-120, mean pore diameter 74 Å, pore distribution ±7:0%, pore volume 0.47 cc/g, surface area 152.7 m²/g), PG500-80 (mesh size 20-80, mean pore diameter 547 Å, pore distribution ±6:3%, pore volume 1.06 cc/g, surface area 44.5 m²/g), PG2000-200 (mesh size 120-200, mean pore diameter 1932 Å, pore distribution ±7:3%, pore volume 0.86 cc/g, surface area 11.32 m²/g). Silica gel 90 (mesh size 80-120, mean pore diameter 74 Å, pore distribution ±7:0%, pore volume 0.47 cc/g, surface area 152.7 m²/g).

Silica gel 60 (Merck) was calcined at 600° C. for 6 h prior to use to obtain hydrophobic silica. All silica and SILP materials were stored in a desiccator over solid NaOH. ¹H NMR spectra were recorded on a Varian Mercury 300 (300 MHz) and are reported relative to the residual solvent peak (δ=4.79 for D₂O and δ=2.50 for d₆-DMSO). ¹³C NMR spectra were recorded on a Varian Mercury 300 (75.5 MHz) or a Varian Inova 500 (125 MHz) and are reported relative to the residual solvent peak (δ=39.52 for d₆-DMSO). FT-IR measurements were carried out with a Bruker ALPHA-P FT-IR spectro-photometer using an ATR platinum with diamond crystal. OPUS 6.5 software was used to analyze the samples. Spectra were taken with 24 scans unless otherwise stated. Structures of synthesized ionic liquids are given in Table 1.

TGA Analysis, General Procedure

Thermo gravimetric analysis (TGA) was performed on a Mettler Toledo TGA/DSC 1 STAR System with a Gas Controller GC 100 and a flow of nitrogen of 70 mL/min. A crucible was weighed inside the TGA instrument and approximately 10 mg of IL or 25 mg of SILP was transferred to it before reinsertion. The weight was noted and during measurements samples were heated from 40 to 600° C. at a constant heating rate of 10° C./min.

Two ways of performing the TGA including recycling (flow absorption-desorption studies) are used in the present recycling studies. The two different TGA methods are illustrated graphically in FIG. 8 (Absorp 3.2) and FIG. 9 (Absorp 4.0). In the absorp 3.2 method the sample is heated to 80° C. in 180 min to remove remaining solvent followed by cooling to 25° C. with CO₂ absorption in 120 min, heating etc. In the absorp 4.0 method the initial heating is conducted at 100° C. and after the cooling to 25° C., the sample is left for 10 min under N₂ before CO₂ is lead over the sample (for 60 min), and intermediate heating steps are performed at 80° C. etc. In both methods the sample starts taking up N₂ during the cooling step. The same is seen with pure SiO₂ so a theory is that all the N₂ uptake stems from SiO2 N₂ uptake.

Synthesis of Tetrabutylammonium Methionate [N4444][Met]—Table 1, entry 1

General procedure 1: 9.72 g of tetrabutylammonium hydroxide ([N4444][OH], 40 wt % in water, 15 mmol) was added to a 50 mL round-bottom flask along with a magnet stirrer. 2.45 g of L-methionine (16 mmol) was added and the edge of the flask was washed with 2 mL of water. The mixture was stirred overnight at rt to ensure full deprotonation of the amino acid. The magnet was removed and the water was evaporated at 10 mbar and 40° C. for 1 h. Excess amino acid was removed by adding 10 mL of cold acetonitrile. The flask was immersed into an icebath and allowed to precipitate for 1 h, followed by separation by a glas frit. The solvent was evaporated at 200 mbar and 40° C. for 1 h followed by 10 mbar and 40° C. for 1 h. ¹H and ¹³C NMR spectra were taken in D₂O and confirmed the identity of the ionic liquid. Purity of the IL was estimated by integration of visible impurities in the ¹H NMR spectrum.

¹H NMR (500 MHz, D₂O) δ=3.40 (dd, J=7.2, 5.6 Hz, 1H), 3.20 (m, 8H), 2.58 (t, J=7.8 Hz, 2H), 2.13 (s, 3H), 1.97 (m, 1H), 1.85 (m, 1H), 1.66 (m, 8H), 1.36 (m, 8H), 0.96 (t, J=7.4 Hz, 12H); ¹³C NMR (75 MHz, D₂O) δ=181.85, 58.19, 55.43, 52.14, 34.29, 30.01, 23.29, 19.35, 14.41, 13.19; IR (ATR platinum diamond 1 Refl): 3359, 3285, 2960, 2935, 2874, 1584, 1487, 1464, 1380, 1326. Yield≧99%. Purity≧99% w/w.

Synthesis of Tetrahexylammonium Isoleucinate [N6666][Ile]—entry 2

Following general procedure 1, tetrahexylammonium isoleucinate ([N6666][Ile]) was synthesised and its identity was confirmed by ¹H and ¹³C NMR.

¹H NMR (300 MHz, D₂O) δ=3.24-3.09 (m, 8H), 3.07 (d, J=5.3 Hz, 1H), 1.64 (br. s, 8H), 1.51-1.23 (m, 25H), 1.23-1.01 (m, 1H), 0.87 (m, 18H); ¹³C NMR (75 MHz, D₂O) δ=181.57, 61.27, 58.21, 38.81, 30.92, 25.47, 24.41, 22.14, 21.31, 15.97, 13.77, 11.68; IR (ATR platinum diamond 1 Refl): 3381, 3288, 2955, 2926, 2860, 1583, 1487, 1465, 1379, 1327.

Yield≧99%. Purity≧99% w/w.

Synthesis of Tetrahexylammonium Glycinate [N6666][Gly]—entry 3

Following general procedure 1, tetrahexylammonium glycinate ([N6666][Gly]) was synthesised and its identity was confirmed by ¹H and ¹³C NMR.

¹H NMR (500 MHz, d₆-DMSO) d=3.69 (m, 8H), 2.66 (s, 2H), 1.57 (br. s, 8H), 1.29 (br. s, 24H), 0.88 (t, J=6.6 Hz, 12H); ¹³C NMR (75 MHz, d₆-DMSO) d=174.40, 57.58, 46.29, 30.60, 25.44, 21.89, 21.01, 13.69; IR (ATR platinum diamond 1 Refl): 3362, 3293, 2954, 2925, 2858, 1593, 1489, 1466, 1378, 1324.

Yield 99%. Purity≧99% w/w.

Synthesis of Tetrahexylammonium Asparaginate [N6666][Asn]—entry 4

Following general procedure 1, tetrahexylammonium asparaginate ([N6666][Asn]) was synthesised and its identity was confirmed by ¹H and ¹³C NMR. ¹H NMR (300 MHz, D₂O) δ=3.56 (dd, J=9.1, 4.7 Hz, 1H), 3.17 (m, 8H), 2.65 (dd, J=14.8, 4.7 Hz, 1H), 2.40 (dd, J=14.8, 9.1 Hz, 1H), 1.64 (br. s, 8H), 1.32 (br. s, 24H), 0.87 (t, J=6.8 Hz, 12H); ¹³C NMR (75 MHz, D₂O) δ=181.26, 176.79, 58.27, 53.44, 40.58, 30.54, 25.30, 21.85, 21.07, 13.36; IR (ATR platinum diamond 1 Refl): 3490, 3363, 3296, 3181, 2954, 2927, 2859, 1667, 1590, 1487, 1466, 1378.

Yield≧99%. Purity≧99% w/w.

Synthesis of Tetrahexylammonium Prolinate [N6666][Pro]—entry 5

Following general procedure 1, tetrahexylammonium prolinate ([N6666][Pro]) was synthesised and its identity was confirmed by ¹H and ¹³C NMR.

¹H NMR (300 MHz, D₂O) δ=3.42 (dd, J=8.1, 6.1 Hz, 1H), 3.16 (m, 8H), 2.99 (m, 1H), 2.70 (m, 1H), 2.04 (m, 1H), 1.58 (m, 11H), 1.29 (br. s, 24H), 0.85 (t, J=6.7 Hz, 12H);

¹³C NMR (75 MHz, D₂O) δ=182.05, 61.63, 58.27, 46.17, 42.99, 30.91, 30.66, 25.35, 21.96, 21.14, 13.53; IR (ATR platinum diamond 1 Refl): 3291, 2954, 2927, 2859, 1590, 1487, 1466, 1373.

Yield≧99%. Purity≧99% w/w.

Synthesis of Tetrahexylammonium Tyrosinate [N6666][Tyr]—entry 6

Following general procedure 1, tetrahexylammonium tyrosinate ([N6666][Tyr]) was synthesised and its identity was confirmed by ¹H and ¹³C NMR. However, for the precipitation of excess amino acid, 10 mL of cold MeOH was used instead of MeCN. This IL is sensitive to light and was therefore wrapped in aluminum foil during synthesis and storage.

¹H NMR (300 MHz, D₂O) δ=6.99 (d, J=8.5 Hz, 1H), 6.61 (d, J=8.5 Hz, 1H), 3.40 (dd, J=7.6, 5.0 Hz, 1H), 3.11 (m, 8H), 2.87 (dd, J=13.6, 5.0 Hz, 1H), 2.66 (dd, J=13.8, 7.6 Hz, 1H), 1.61 (br. s, 8H), 1.30 (br. s, 24H), 0.81 (m, 12H); ¹³C NMR (75 MHz, d₆-DMSO) d=176.63, 160.67, 129.51, 126.61, 116.09, 58.45, 57.64, 42.28, 30.62, 25.46, 21.92, 21.01, 13.83; IR (ATR platinum diamond 1 Refl): 3233, 2954, 2926, 2859, 1589, 1513, 1486, 1466, 1380. Yield≧99%. Purity≧96% w/w.

Thermal Stability of SILP Absorbers

The ILs of particular interest according to the present invention are ones with a capability of capturing CO₂ from a dilute stream, such as off-gas from combustion, and concurrent release of the gas by temperature swing absorption (TSA). For this purpose the amino acid functionalized ILs according to the present invention and their corresponding SILP materials were synthesized and analyzed for temperature stability.

The temperature stability of the ILs was estimated by TGA, and the point of decomposition was determined by initial curvature of both the curve representing the weight as well as the heat flux to the sample. The points of decomposition for all the pure ILs were measured to be 140-150° C. except for [N6666][Tyr] which decomposed at approximately 130° C. After impregnation on SiO₂ (see below example) the resulting SILP materials showed comparable or slightly higher temperature stabilities vis-a-vis their IL counterparts, thus making them applicable for TSA.

Synthesis of Hydrophobic Silica

Approximately 15 g of SiO₂ (marked Silica 60) was calcined at 600° C. for 6 h. The hydrophobic silica was stored in a desiccator over NaOH(s). This hydrophobic silica is used as a support material in the following.

Synthesis of Supported Ionic Liquid Phase Absorber Materials

2 g of [N6666][Met] was added to a 100 mL round-bottom flask along with a magnet stirrer. 10 mL of MeOH was added followed by 3 g of hydrophobic silica. The suspension was stirred overnight. The magnet was removed and the solvent was evaporated at 330 mbar and 40° C. for 1 h followed by 10 mbar and 40° C. for 1 h. The sticky solid was weighed and transferred to a glass fritted funnel and Ar was flowed through it for approximately three days at approximately 70 mL/min to remove remaining solvent. The dry SILP was transferred to a small container and stored in a desiccator over NaOH(s). All other examined SILP absorbers with 10-40 wt % IL loading were prepared by a similar method as described above (see Table 2).

The used carrier material, SiO₂, was chosen due to easy access and convenient calcination. It also has a high surface area and a mesoporous structure, both of which are important for a support material. Other support materials relevant in the context of the present invention include controlled pore glasses, alumina, titania (anatase), zirconia, alkaline earth metal oxides, clays and zeolites, or mixtures hereof.

CO₂ Absorption Studies

CO₂ Absorption Capacities of the SILP Absorbers:

The equilibrium CO₂ absorption capacities of the SILP absorbers were determined under 1 bar of CO₂ pressure using the following method: A glass reactor was dried in an oven at 120° C. overnight, assembled hot, and allowed to cool for approximately 1 h. After weighing 1-2 g of SILP was transferred to the reactor where after it was degassed with helium (approximately 70 mL/min) for 24 h or until no further weight loss was observed. Subsequently, the reactor was subjected to a stream of approximately 50 mL/min of CO₂ gas (99.9%, Air Liquide Denmark) and the weight increase of the reactor followed periodically for 48 h. The absorption dynamics using a SILP absorber was demonstrated in a flow-setup at 0.09 bar CO₂ pressure (in He) using the following method: 0.78 g of a SILP absorber with 40 wt % [N6666][Pro] was loaded between glass wool into a glass tube reactor with inner diameter of 1 cm. The system was then purged with helium (50 mL/min) for approximately 30 min, where after a flow of CO₂ (5 mL/min) was directed into the helium stream and the CO₂ concentration at the reactor outlet was continuously measured using a CO₂ BINOS 100 detector (Rosemount Analytical, Solon, Ohio, USA). A blank experiment was also performed without absorber material in the reactor.

The absorption measurements revealed a number of the prepared amino acid-based SILP absorbers to provide excellent CO₂ uptake, as shown in Table 1 and Table 2. Hence, many of the SILP materials absorbed superstoichiometric amounts of CO₂, which is surprising considering the very slight physical solubility of CO₂ in ILs in general, and specifically at ambient pressure. All superstoichiometric uptake amounts are assigned to physical absorption (physisorption) as the ILs of the present invention are only able to bind one mol CO₂ by a chemical reaction with the amino group. Accordingly, the ILs of the invention very likely have accessible cavities for CO₂ to intercalate, possibly aided by attraction to the negatively charged carboxylate moiety present in the amino acid anions.

To determine if the superstoichiometric uptake had anything to do with the support porosity, specifically trapping of CO₂ or IL in its pores, a series of lower loading SILP materials based on [N6666][Pro] was synthesised. After synthesis they were not characterised, only dried and submitted to CO₂ absorption experiments (Table 3).

The SILP material with 10 wt % [N6666][Pro] loading showed a particular high equilibrium absorption capacity of 1.74 mol CO₂ mol IL (at 25° C., 1 bar CO₂). At higher loadings this ratio decreased gradually, as expected, indicating that the support affected the capacity although absorption by the SiO₂ was subtracted in the reported data. ILs have a high degree of ordering in the liquid state [^(a)Jacquemin J, et al. High-pressure volumetric properties of imidazolium-based ionic liquids: Effect of the anion. J Chem Engineer Data, 2007, 52: 2204-2211; ^(b)Huang X et al. Why is the partial molar volume of CO2 so small when dissolved in a room temperature ionic liquid? J.Am Chem Soc, 2005, 127: 17842-17851], and this ordering might be further promoted by impregnation on a solid surface [^(a)Gottfried J M et al., Surface studies on the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate using X-ray photoelectron spectroscopy (XPS). Z für Physikalische Chem, 2006, 220: 1439-1453; ^(b)Steinrück H-P. Recent developments in the study of ionic liquid interfaces using X-ray photoelectron spectroscopy and potential future directions. Phys Chem Chem Phys, 2012, 14: 5010-5029]. The surface-induced ordering of the IL would then extend some distance into the bulk liquid, and the effect is therefore more pronounced with low loading.

To further assess the CO₂ absorption abilities of the SILP materials, flow absorption-desorption studies (recycle studies) were performed using the 40 wt % [N6666][Pro] SILP absorber, which possessed the highest CO₂ mass uptake, with a gas stream of 0.09 bar CO₂ (i.e. 9 mol %) in He. The result showed that insertion of the amino acid-based SILP material into the CO₂-containing stream led to a fast CO₂ gas uptake and a “silent” time of approximately 50 sec before breakthrough compared to a blank run without absorber material, see FIG. 4. This corresponds roughly to utilization of one third of the SILP material's maximum gas uptake capacity. Consecutive absorption-desorption cycles for the 40 wt % [N6666][Pro] SILP absorber (desorption done at 80° C. for 2 h under argon flow) demonstrated further complete desorption between each sorption cycle, and preservation of good absorption capacities of 1.05 and 1.17 mol CO₂/mol IL for the second and third cycle, respectively (see table 4). For CO₂ uptake the last column should be examined, as it has been corrected for uptake of CO₂ by SiO2, while for the desorption steps, the third column should be examined. This number has not been corrected for uptake of CO₂ by the support material and will therefore be zero when the initial weight of the sample has been reached. As seen in table 4, the CO₂ uptake becomes negative at the second and third desorptions. As the subsequent uptake of CO₂ is superstoichiometric, this is probably not due to decomposition or evaporation of the active compound, but more likely to be desorption of solvent or moisture captured in the SILP material.

Consecutive absorption-desorption cycles for the 40 wt % [N6666][Gly] SILP absorber (desorption done at 80° C. for 2 h under argon flow, see FIGS. 10 a and 10 b) demonstrated further complete desorption between each sorption cycle, and preservation of good absorption capacities for the second, third and fourth cycle, respectively (see table 5). A very small loss of capacity is seen from the first to subsequent cycles. The absorption stoichiometry is around 1.00 mol of CO₂ per mol IL in this material. Absorption kinetics is very fast.

The recycle studies conducted for, [N6666][Gly] can be seen in FIG. 10. FIG. 10 a is raw data where the background is “drifting”. This is probably due to evaporation of solvent. The graphs are normalized so that the lowest point in the first cycle has the value 0 and the rest is calculated from there which means that the lowest point in the following cycles are below 0. FIG. 10 b is corrected for this shifting, and is parallel displaced upwards so that all the cycle graphs begins in 0. The scale has been recalculated to a fraction of theoretically maximal CO₂ uptake by the SILP material.

These results clearly demonstrate the potential to use amino acid derived SILP absorbers for post-combustion CO₂ capture at relevant gas concentration levels by TSA operation.

FT-IR Analysis, General Procedure

A background spectrum was taken and sample was transferred to the ATR crystal to completely cover it. For liquid samples, a few drops were used, and for solid samples approximately 10 mg was used and pressed against the ATR crystal using the pressure arm. All spectra were recorded with 24 scans and corrected for background noise before analysis.

The active part for CO₂ absorption in the synthesized SILP materials is the amino acid anions. Thus, upon uptake of CO₂ some noteworthy changes in the IR spectrum of the anions are expected if carbamate formation occurs, as also previously shown by Brennecke et al. [Gurkan B E et al. Equimolar CO₂ absorption by anion-functionalized ionic liquids. J Am Chem Soc, 2010, 132: 2116-2117].

In all the FT-IR spectra of the SILP materials before and after CO₂ absorption, significant changes in both the N—H stretching region and/or the 1700-1300 cm⁻¹ region were observed (see exemplary FIG. 5-7 for [N6666][Asn]). This leads to the conclusion that CO₂ indeed was chemically absorbed up to 1 mol CO₂ per mol IL via the nitrogen moiety of the amino acid anions in the SILP materials, as expected.

FIG. 5-7 were recorded for [N6666][Asn] and SILP versions thereof. For the neat IL [N6666][Asn] itself four N—H stretch bands are seen in the spectrum, while only two poorly resolved bands are seen in the SILP spectrum. After CO₂ absorption, one of these bands has become strong. The 1700-1300 cm⁻¹ region also shows clear signs of chemical uptake of CO₂ with the appearance of a C═O stretch at 1646 cm⁻¹ and a band at 1581 cm⁻¹ that might come from a now visible N—H bending motion. 

1. A composition for absorption/desorption of gaseous carbon dioxide (CO₂), which comprises an ionic compound [A⁺][B⁻] and a porous material, wherein [A⁺] is an ammonium ion of Formula I

wherein R₁, R₂, R₃ and R₄ are individually selected from linear or branched C_(n) alkyl chains where n is an integer having a value of at least 4 and not more than 30, and wherein [B⁻] is an anion of Formula II:

wherein R₅ is selected from hydrogen or a C₁-C₆ linear or branched alkyl group, and G can be hydrogen or a group of Formula III:

wherein R₆ is selected from hydrogen or a C₁-C₆ linear or branched alkyl group; R₇ can be a C₁-C₆ linear or branched alkyl group or an aromatic group, and when R₅ is hydrogen, R₇ may further also be attached to the nitrogen atom to which R₅ is attached, thereby forming a 4-7 membered ring together with said nitrogen.
 2. A composition according to claim 1 wherein the ionic compound [A⁺][B⁻] comprises an anion of one of the following amino acids:


3. A composition according to claim 1 wherein the ionic compound [A⁺][B⁻] comprises one of the following ammonium ions:


4. A composition according to any one of the preceding claims wherein the porous material is selected from porous (including mesoporous) inorganic, carbonaceous or a polymeric materials such as, but not limited to, silica, including mesoporous silica, controlled pore glass, alumina, titania (anatase), zirconia, alkaline earth metal oxides, clays and zeolites, or mixtures hereof.
 5. A composition according to any one of the preceding claims which contains between 5-45% w/w of the ionic compound [A⁺][B⁻], such as 10% w/w, 20% w/w, 30% w/w or 40% w/w.
 6. A composition according to any one of the preceding claims which contains between 5-45% w/w, such as 10% w/w, 20% w/w, 30% w/w or 40% w/w, of an ionic compound selected from [N4444][Met], [N6666][Ile], [N6666][Gly], [N6666][Asn], [N6666][Pro] and [N6666][Tyr].
 7. A composition according to 6 which contains 40% w/w of an ionic compound selected from [N6666][Ile], [N6666][Gly] and [N6666][Pro].
 8. A method for absorption of gaseous CO₂ from a gas stream, which method comprises contacting said gas stream with a composition according to any one of the preceding claims.
 9. A method for absorption of gaseous CO₂ from a gas stream according to claim 8, wherein the absorption takes place from a gas stream which contains 1-100 mol % CO₂ with an absorption capacity of over 1 mol CO₂/mol of the ionic compound [A⁺][B⁻].
 10. The method according to any one of claims 8-9 wherein the contact between the gas stream and the composition takes place in a suitable reactor such as a fixed-bed or movable-bed reactor using an appropriately shaped form of the composition which provides low resistance to the gas stream and an acceptable pressure drop across the reactor.
 11. The method according to any one of claims 8-10 wherein the gas stream is a flue-gas stream generated from combustion by large point stationary sources such as power plants.
 12. A method for desorption of CO₂ absorbed on a composition according to any one of claims 1-7, which method comprises increasing the temperature of the composition and/or decreasing the total pressure surrounding it and/or by flushing it with a gas stream with no or lower CO₂ content than the gas stream originally applied for the absorption. 